Treatment of tungsten filaments.



CARL T. FULLER,

OF N UTLEY, NEW JERSEY, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

TREATMENT OF TUNGSTEN FILAMENTS.

1,280,704. No Drawing.

To all whom it may concern:

Be it known that I, CARL T. FULLER, a citizen of the United States, residing at Nutley, county of Essex, State of New J ersey, have invented certain new and useful Improvements in the Treatment of Tungsten Filaments, of which the following is a specification.

My present invention relates to a method of treating tungsten to increase its ductility and pliability and improve its capacity for retaining those properties after heating to the high temperatures of incandescence commonly employed in incandescent lamps and similar devices.

It has been found that tungsten in the form of wire when prepared by the usual methods of manufacture heretofore employed, although ductile and pliable when sten wire is heated in the presence of an oxidizing agent under certain onditions it becomes so pliable as to readily take a permafient set without heating and that it retains its ductility for a longer period after.

heating to incandescence. The exact nature of the action which takes place whereby the improved qualities of the wire are secured is not definitely known at the present time. It seems very likely however, that the oxygen acts to remove from the wire various impurities found therein in small amounts. These impurities may be present in the original metal and hence scattered throughout the entire mass or they may have been absorbed during the process of manufacture and confined .to a thin surface layer of the wire.

In carrying my invention into effect if the wire is to be used for filaments, I may treat it after it has been mounted in the lamp by passing current through the lamp while it is connected to the evacuating pum andbefore the degree of vacuum at whic the lamp is ordinarily sealed off has been reached. The small amount of oxygen prescut is suflicient to efl'ect the desired improvement. Preferably, however, the completed- Specification of Letters Patent.

Patented 0ct.8,1918.

Application filed April 13, 1914. Serial No. 831,411.

lamps as more uniform results are obtained in this way. In this case the process may be made continuous by carrying the wire over two contacts in a partial vacuum or. in an attenuated atmosphere of oxidizing gas and passing current through it between the contacts to heat it to the desired temperature. It will also be understood that this latter method of treatment will be applicable when the wire is to be used for other purposes than for filaments.

In the manufacture of filaments ofductile tungsten for incandescent lamps, diificulty has been experienced due to the tendency of short sections of the filament to move laterally upon each other after prolonged heating to incandescence and produce the condition known as off-setting. Various methods of preparing tungsten have been devised, however, whereby this effect has been avoided. For example, it has been found that if pure tungstic oxid is heated in a gas furnace in a covered Hessian or Battersea crucible it absorbs a small quantity of matter consisting largely or alumina and silica which probably is volatilized from the crucible. The presence of from .8 to 1.5 of one per cent. of this additional matter introduced at this stage of the refining of tungsten prevents offsetting of the completed filament. It has also been found that the same result maybe attained by adding to the tungsten small quantities of various refractory materials such as thorium oxid or thorium nitrate. My invention is especially effective in connection with tungsten wire which has been prepared by a process which results in non-offsetting filaments.

The most suitable temperature for producing the desired results, the amount of the oxidizing. agent necessary and the length of time of heating the wire will vary. Generally speaking it is believed that these factors depend upon the size bf the wire and the physicaland chemicalcharacteristics of the wire before treating and will have to be determined experimentally in each case. In order to enable one skilledin the art to practise my invention I will state however, that with a wire having a diameter of 1.2 mils which had been rendered non-offsetting by the addition of .75% of thorium oxid to the tungstic oxid from which it was produced I have obtained satisfactory results by passing the wire over two contacts spaced,

one foot apart in a vacuum oil-2 mm. of mercury at a rate of 10 feet per minute, and heatin under these conditions to'a temperature of approximately 1500 degrees 0. Wire thus treated, in addition to being less liable to be broken during the manufacture of the lamp, retains its ductility throughout a greater portion of the useful life of the lamp than wire which has not been subjected to this process.

I have also obtained good results by firs oxidizing the wire slightly, as for example by heating it to a dull red heat in the open air, and subsequently heating the oxidized wire to a high temperature in a good-vacuum, such as 1/10 mm. of mercury. The oxidized wire may be heated in an evacuated.

furnace or chamber, or it may be mounted to form the filament of an incandescent lamp and then heated when the vacuum becomes sufliciently good during the exhaustion of the lamp. The temperature'of the wire in the vacuum is increased gradually so thatany tungsten oxid which is present will decompose without vaporizing or causing a dark deposit on the lamp bulb.

-I have also foimd that good results are obtained if tungsten wire suitably oxidized, as by heating in the open air, is subsequently heated to the proper temperature in a suitable gas, such as nitrogen or forming as, which is a mixture of nitrogen and hy rogen. If heated too hot while in the forminggas the wire may become brittle. I have found that the oxidized wire can be heated much higher with safety and that better results are obtained if the nitrogen or forming gas contains some air, or a large amount of water vapor, which may be introduced by bubbling the forming gas through water. 40

It will of course be understood that my invention is not limited to the treating of tungsten wire for use as filaments for incan descent lamps but that it will be of value in any case in whichtungsten wire is to be '45 used for purposes where great strength, ductility and pliability are desirable and also where it is essential that these properties should be retained after prolonged heating to high temperatures. 6

What Iclaim as new and desire to secure by Letters Patent of the United States, is

1. The process of improving the mechanical properties of drawn tungsten wire which has been drawn by the usual methods of 86 manufacture and which is later to be subjected to a temperature substantially hi her than red heat which consists in heating 1t to at]:1 least a red heat in an'oxidizing atmosp ere. I

2. The process of treating drawn tungsten which consists in heating it to a high temperature in an attenuated atmosphere of an oxidizing agent.

In witness whereof I have hereunto-set my hand this 10th day of April, 1914. a

' CARL T. FULLER.

Witnesses:

S. N. WHITEHEAD, v Fnamr Parsons. 

